This invention relates to a throwaway cutting tool such as a throwaway rotary cutting tool e.g. a drill or a throwaway tool. The throwaway cutting tools used here mean head exchangeable tools but include a type of tools in which a cutting edge formed on the cutting head is reground and the cutting head is changed when such regrinding becomes impossible.
Among known throwaway rotary cutting tools, there is one disclosed in JP patent publication 11-504269. With the cutting tool of this publication, a cutting head (cutting portion) having a cutting edge is detachably coupled to the tip of a tool body having a shank by applying a clamping force with a pull rod to allow regeneration of functions by replacing the cutting head. Protrusions and recesses are formed opposite to each other on the front surface of the tool body and a support surface of the cutting head to be brought into abutment with the former surface to obtain a wedging effect, and the protrusions and recesses are fitted together to transmit torque, and also to increase the clamping force between the tool body and the cutting head when the thrust force applied to the tool increases.
Besides, there is one in which, as in JP patent publication 2001-503683, protrusions and recesses for fitting are provided in an X arrangement on two imaginary lines that intersect at an acute angle on the center of rotation.
With the tool disclosed in JP patent publication 11-504269, since the directions of the protrusions and recesses provided on the front surface of the tool body and the support surface of the cutting head are unified in one direction, torque transmission during cutting is done only through some of the protrusions and recesses, so that stress concentration occurs at such some of the protrusions and recesses. Thus, endurance problem tends to occur particularly during high-load cutting.
Also, dimensions of the cutting head made of cemented carbide (pitch of the protrusions and recesses provided on the support surface) vary due to delicate change in shrinkage factor due to variations in the sintering conditions, so that adhesion of the cutting head to the tool body worsens. This will worsen runout accuracy during setting and thus have a bad influence on the machining accuracy and cutting performance.
Further, in machining an oblique hole with a drill, lateral load is applied to the cutting head. By bearing this load on the pull rod, the cutting head is prevented from moving. But since the pull rod is at the center of rotation, which is apart from the load-application point (outer periphery) on the cutting head, the moment applied to the pull rod increases. Besides, since the pull rod has a long length and the absolute amount of elongation tends to increase due to pulling, when it is subjected to a lateral load, the pull rod is drawn, so that fixing of the cutting head becomes unstable, which may lead to chipping of the cutting edge.
In contrast, with the tool of the JP patent publication 2001-503683, since the protrusions and recesses are provided on the imaginary lines in a cross arrangement with reference to the central axis of rotation, lowering of the fitting accuracy due to dimensional change resulting from unbalanced transmission of cutting torque or heat shrinkage decreases compared with the above-described prior art. But since the protrusions and recesses are simple and small in number, reliability of torque transmission and durability are insufficient. Also, since coupling is done by means of a pull rod, rigid fixation of the cutting head cannot be desired.
With cutting tools used with workpieces rotating, such as boring bars, there are ones in which a cutting head (cutter tip) is mounted to the tip of a tool body (shank) by protrusion-to-recess fitting, as in German patent No. 3448086.
Among these tools, there are ones in which V-fitting recesses and protrusions are provided at three locations in a radial arrangement to clamp the cutting head with six surfaces. But this cannot insure centering accuracy and clamp stability unless the fitting surfaces are finished with high accuracy. Also, since the cutting force is borne at three locations, the recesses and protrusions tend to be fatigued or damaged. On the other hand, there are ones in which one square shaft and one square hole are formed at the axial portion and fitted together. But some clearance is left between the square shaft and the square hole, so that the cutting head (insert) tends to move by the cutting force by an amount of the clearance, so that the cutting edge tends to chip.
In order to solve the above problems, according to this invention, a plurality of protrusions and recesses extending from the center of the tool toward its outer periphery and having V-shaped sections are provided radially and alternately with each other on a front surface of the tool body and a support surface of the cutting head which is brought into abutment with the front surface. Also, the protrusions have their height gradually increasing and the recesses have their depth gradually increasing from the center of the tool toward its outer periphery.
With a tool having a sufficient space, the cutting head is fixed by two bolts. Specifically, the cutting head is formed with two bolt holes with seats that are parallel to the axis so as to extend from the front surface of the cutting head through the support surface thereof on both sides of the center of the tool of the cutting head, and the cutting head is clamped to the tool body by bolts inserted through the bolt holes.
With a tool having an oil hole such as a drill, an oil hole is provided in the tool body and the cutting head at the center thereof so as to branch in the cutting head and the branched portions of the oil hole communicate with the bolt holes with the outlets of the branched portions of the oil hole formed by the inlets of the bolt holes.
With a tool having in its outer periphery two helical flutes that are symmetrical with respect to the center, an oil hole is formed in the tool body and the oil hole extends through the axis of the tool body and branches in a V-shape in the tool body. Branched portions of the oil hole open to the intersection of the front surface and outer peripheral surface of the tool body near a heel, and oil grooves opening to the helical flutes are formed in the outer periphery of the cutting head by cutting out the heel so as to communicate with the outlets of the branched portions of the oil holes.
With a tool having no cutting edge at the center of the tip, the cutting head may be fixed to the tool body with a single clamping screw passed through the center of the cutting head. For a small-diameter tool, which has little room for the space, this structure is suitable.
Besides, for a tool having at least the cutting head formed of a sintered material, the sintered skin on the engaging protrusions on the cutting head may remain without being removed.
This invention is also applicable to a gun drill for forming deep holes. In this case, a tool body is formed which has a section perpendicular to the axis that is a xc2xe circle and a straight groove in its outer periphery and a plurality of protrusions and recesses of V-shaped section extending from the center of the drill toward its outer periphery are provided radially and alternately with each other on a front surface of the tool body and a support surface of the cutting head. The protrusions have their height gradually increasing and the recesses have their depth gradually increasing from the center of the drill toward its outer periphery. The protrusions and the recesses are arranged radially and have their height and depth varying in a radial direction.
With the throwaway gun drill, a fixing bolt formed with an oil hole along its axis is inserted into a bolt hole with a seat formed in the cutting head at a position offset from the axis of the cutting head and tightened into the support member to couple the cutting head to the support member. The bolt hole is in communication with a cavity of the pipe portion through the oil holes provided in the fixing bolt and the support member. Advantageously oil holes are separately formed in the support member and the cutting head so as to communicate with the cavity in the pipe portion, or the groove surface of the straight groove formed in the cutting head may be higher than that of the straight groove formed in the tool body.
Further, for both gun drills and other tools than a gun drill, a coating film of a hard material may be formed on the surface of the cutting head, or the cutting edge of the cutting head may be formed of sintered diamond or cBN sintered material.
Since the engaging protrusions and recesses provided on the front surface of the tool body and the support surface of the cutting head so as to correspond to each other are radially arranged around the tool center, torque during cutting is transmitted while acting on the respective protrusions and recesses in a direction perpendicular to the longitudinal direction, so that stresses applied to the protrusions and recesses are distributed. This suppresses e.g. fatigue of the fitting portions and chipping of the protrusions due to stress concentration.
Since the height and depth of the protrusions and recesses, which have V-shaped sections, have their height and depth varied so as to increase toward the outer diameter of the tool, their widths narrow gradually toward the tool center. Thus, by increasing the number of the protrusions and recesses, it is possible to markedly increase the area of the fitting surfaces through which torque is transmitted. Also, while the transmission torque is larger on the large-diameter side of the tool than at the center of rotation, if the height and depth of the protrusions and recesses are varied so as to increase toward the outer diameter, the allowance for fitting is larger toward the outer diameter, so that torque can be transmitted ideally. Thus, it is possible to effectively suppress fatigue of the fitting surfaces and prevent chipping of the protrusions.
Also, due to the radial arrangement, even if the shrinkage factors at different portions should vary according to change in conditions during sintering, the pitch of the protrusions and recesses would not be affected. Thus, even if the sintered skin of the protrusions and recesses is left as it is by omitting cutting to lower the cost, the adhesion between the tool body and the cutting head would not worsen. Further, by varying the height of the protrusions and the depth of the recesses in a diametrical direction, the front surface of the tool body and the support surface of the cutting head conically fit. This also produces the centering function, thus increasing the run-out accuracy during setting.
Besides, with the one in which the cutting head is fixed by two bolts provided on both sides of the tool center, burden applied to the bolts due to formation of an oblique hole is distributed. Also, since the bolts are located on the outer-diameter side not at the tool center, the moment is smaller, so that drawing of the bolts due to lateral loads is suppressed, thus increasing stability of fixing.
Also, with the one in which the two bolt holes are also used as outlets of the oil holes, internal oil supply is possible by providing oil holes even in a small-diameter tool having a small extra space.
With the one in which the oil hole is branched in the tool body and open to the intersecting portion of the front surface and the outer periphery of the body near the heel, cutting oil blown out from the branched oil holes flows through the oil grooves formed in the outer periphery of the cutting head into the helical flutes to which the oil grooves open, so that cutting oil can easily reach the cutting edge. Also, by increasing the sectional area of the branched oil hole or oil grooves, the discharge pressure decreases, so that it is possible to increase the effect of flowing chips away with cutting oil by supplying a large amount of cutting oil. Further, since there is no need to provide an oil hole in the cutting head, the head can be easily manufactured. This is advantageous in the cost too.
With a gun drill, if the supply amount of cutting oil is insufficient, the life of the cutting edge will be short, and such problems as clogging with chips tend to arise. Thus, the number of fixing bolt used as oil holes should be one and an oil hole should be formed separately. The diameter of the special oil hole can be made larger in diameter than the oil hole formed in fixing bolts, so that it is possible to increase the amount of cutting oil supplied to the cutting edge by providing such a special oil hole.
Also, if the groove surface of the straight groove formed in the cutting head is provided higher than that of the straight groove formed in the tool body, chip will be caught by the joint.